One type of offshore installation utilized to moor a tanker and connect a hose to it, includes a buoy assembly extending from the seabed to the sea surface and with its upper end tied by a hawser to the vessel, and a floating hose structure which extends at an upward incline from the seabed to the surface independently of the buoy assembly. The buoy assembly includes at least one tilt joint which allows it to tilt to follow limited movement of the vessel towards and away from the center of the installation. Both the buoy assembly and hose structure require joints that permit unlimited rotation about a substantially vertical axis, to follow drifting of the vessel about the center of the installation.
One common approach to permit rotation of the buoy assembly and hose structure about the same vertical axis, is to utilize an annular fluid swivel with a large hole in the center through which the buoy assembly extends. This permits the upward mooring load to be transmitted to the seabed without being transmitted directly through the fluid swivel. One disadvantage of this arrangement is that the annular fluid swivel cannot be removed without detaching the buoy assembly from the seabed. In addition, an annular fluid swivel is substantially more complex than a fluid swivel that does not require a hole along its axis of rotation.
One type of mooring installation that facilitates replacement of the fluid swivel while permitting a simple swivel to be utilized, positions the fluid swivel along the vertical axis of rotation, in a hollow portion of the buoy assembly. The buoy assembly has a side opening through which a pipe from the rotatable portion of the fluid swivel can pass sidewardly away from the buoy assembly. British Pat. No. 1,524,906 shows a few installations of this type, wherein the fluid swivel is located on the upper rotating part of the buoy installation where an opening can be provided for the sidewardly extending pipe. However, these designs involve the use of fluid-carrying tilt joints, such as flexible hoses, immediately below the fluid swivel, to carry fluid across a tilt joint of the buoy assembly. Such fluid tilt joints that are subjected to repeated tilting, are liable to breakdown and increase the maintenance requirements for the installation, as where a short section of a flexible hose is used which undergoes repreated considerable bending. A mooring installation of the type which has a hose structure extending to the vessel independently of much of the buoy assembly, which enabled a simple and easily accessed fluid swivel to be utilized without requiring the use of high maintenance underwater parts, would facilitate the construction and operation of mooring and cargo transfer installations.